Microwave devices typically include a plurality of components that may have different characteristic impedances. In order to propagate the microwave signal through the device with minimal loss, the impedances of the various components are matched to the characteristic impedance of the input and output signal. By transitioning the impedances so that an input transmission line is matched, most of the available power from the input is delivered to the device. Historically, impedance matching techniques have treated the matching of components with constant characteristic impedances to a constant characteristic impedance of the input line, e.g. to 50 Ω. Multi-stage matching circuits have been utilized to obtain minimal reflection loss over a specified frequency range of operation of a device. Numerous techniques, such as the use of radial stubs, quarter wave transformers, and multi-stage matching circuits with specific distributions, such as Binomial or Tchebychef, etc., have been developed in order to achieve maximum power transfer from the input to the device.
However, the characteristic impedance of the tunable components in tunable microwave devices is not a constant value. The characteristic impedance of the tunable component varies over the operating range of the device from a minimum to a maximum impedance value. In tunable dielectric devices, a bias voltage applied to tunable dielectric material provides the ability to alter the dielectric constant. The change in the dielectric constant provides a variation in the electrical path length of a microwave signal. As the electrical properties of the tunable dielectric material are varied, the characteristic impedance is also affected.
In practice, a single characteristic impedance within the tunable components minimum/maximum impedance range is selected. This single impedance value is matched using one of the state of the art impedance matching techniques. However, as the tunable microwave device is operated, the impedance of the tunable component varies from the matched impedance and a degradation in the impedance match occurs.
Prior tunable dielectric microwave transmission lines have utilized tuning stubs and quarter wave matching transformers to transition the impedance between the input and output. The technique is best for matching a fixed impedance mismatch. U.S. Pat. No. 5,479,139 by Koscica et al. discloses quarter wavelength transformers using non-tunable dielectric material for the purpose of impedance matching to a ferroelectric phase shifter device. Similar impedance matching configurations using non-tunable dielectric substrate of background interest are shown in U.S. Pat. No. 5,561,407, U.S. Pat. No. 5,334,958, and U.S. Pat. No. 5,212,463. The disadvantage of the above technique is that the impedance match is optimal at one selective tuning point of the device and degrades as the device is tuned through its range. Hence, the reflection loss due to impedance match increases when the device is tuned away from the matched point.
Another impedance matching approach for tunable devices is presented in U.S. Pat. No. 5,307,033 granted to Koscica et al. That patent discloses the use of spacing of a half wavelength between elements or matching networks for the purpose of impedance matching.
Still another approach utilizes quarter wavelength transformers on tunable dielectric material as disclosed in U.S. Pat. No. 5,032,805, granted to Elmer et al. Other impedance matching configurations are shown in U.S. Pat. Nos. 6,029,075; 5,679,624; 5,496,795; and 5,451,567. Since it is also desirable to reduce the insertion loss of the matching network, a disadvantage of the above approach is that the quarter wavelength transformer on tunable dielectric material increases the insertion loss.
The disclosures of all of the above-mentioned patents are expressly incorporated by reference.
It would be desirable to minimize the impedance mismatch in tunable microwave device applications. There is a need for a technique for improving impedance matching for tunable microwave components that achieves minimal reflection and insertion losses throughout the range of operation of tunable devices.